Facing Goya is Michael Nyman's second opera. His first, The Man Who Mistook His Wife For A Hat, based upon a case-study by Oliver Sacks, appeared in 1986, but despite the stream of concert works and film scores he has produced since then, there have been no stage works, until now.

The new piece, with a libretto by playwright Victoria Hardie, was premiered in Santiago de Compostela last week and is now touring Spain. A British venue has yet to be organised.

The starting point for Hardie's text is a fictional search for the whereabouts of Goya's skull, which the painter had, in fact, asked to be removed from his corpse before burial. Goya did not want his head falling into the hands of the early eugenicists and craniometrists, who, he felt, might use it to bolster their pernicious theories about racial difference and the determination of creative genius through measuring the dimensions of the human skull and brain.

The action begins in the 19th century, as a woman searches for the skull, hoping to confound all the contemporary theories about craniometry and to confirm Goya's fears about such pseudo-science. Then it fast-forwards to the 1930s, where eugenics as a benign form of social engineering was corrupted by the Nazis, and finally on to the revolutions in gene manipulation of the last 20 years.

Goya's skull is discovered, sold to a bio-tech corporation, and his DNA used to produce the first human clone. But though the youth who results looks like Goya, he lacks his genius.

The text is complex and rather too wordy. Conventionally in opera it is the characters who develop; their personal journeys fuel the drama. In Facing Goya, ideas dominate, and the way in which they change through time while raising perennial moral issues provides the work with its dynamic. The cast of five singers - together with two actors who present the gist of each of the opera's four parts in the language of the audience - play stereotypes; they're more important for what they represent than for who they are. Complex arguments are presented, scientific references abound. The course of the debate is sometimes hard to follow and sometimes stubbornly undramatic. The pacing of the second half, leading up to the revealing of the Goya clone, is tighter and more compelling than the first, though the last 20 minutes of this almost three-hour show needs to be considerably more purposeful.

Yet Nyman's score supports the words wonderfully: it's one of his finest achievements. The propulsive style, with its amplified orchestra and singers - which would probably sound more incisive in an indoor theatre than it did in the open-air at the Perelada festival near Barcelona, where I saw it - is instantly recognisable; while his extraordinary skill at rediscovering the expressive power of the common chord, or reinventing the dramatic potency of a simple cadence, provides him with a rich variety of musical possibilities.

There are some insistently memorable melodic lines and, for a composer who has always prided himself on his magpie tendencies, very few explicit borrowings, though the soaring tune that dominates Beethoven's third Leonora overture is recruited to announce the appearance of Goya in the final scene. It does the job superbly.

At times Nyman also makes a virtue out of the text's lack of dramatic direction by allowing his music to take charge. Without the constraints of following a scenario, the score can then visit some exotic locations. These have been eagerly seized upon by Jonathan Moore in his staging, so that the action, often necessarily rather studied, can erupt into physicality. Moore has done a very fine job in presenting the work and its arguments, with Conor Murphy's designs relying heavily on projected images; details of Goya's paintings, craniometric projections, Riefenstahl's images of the 1936 Berlin Olympics, DNA fingerprints and genome maps spill across the stage.

The cast, conducted by Harry Lyth and supported by the Michael Nyman Band, are first-class too. The characters played by the contralto Hilary Summers, from a 19th-century art historian to a 21st-century "art banker", articulate the works and Goya's moral consciousness, while Winnie Bowe, Marie Angel, Harry Nicoll and Omar Ebrahim take on the array of scientists, eugenicists, critics and businessmen. It's a fascinating work, and needs to be seen in Britain as soon as possible.